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          <h1 class="post-title" itemprop="name headline">一探究竟源计划--String</h1>
        

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        <h3 id="String类的定义"><a href="#String类的定义" class="headerlink" title="String类的定义"></a>String类的定义</h3><figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">public</span> <span class="keyword">final</span> <span class="class"><span class="keyword">class</span> <span class="title">String</span> <span class="keyword">implements</span> <span class="title">Serializable</span>, <span class="title">Comparable</span>&lt;<span class="title">String</span>&gt;, <span class="title">CharSequence</span> </span>&#123;</span><br><span class="line">  ……</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>
<ul>
<li>String类是final的，不可以被继承，主要是为了“效率”和“安全性”的缘故。若String允许被继承，由于它的高度被使用率，可能会降低程序的性能</li>
<li>String字符串是常量，其值在实例创建后就不能被修改</li>
<li>String是基于字符数组char[]实现的</li>
</ul>
<p>String、StringBuffer与StringBuilder的区别:</p>
<ul>
<li>String类型和StringBuffer类型的主要性能区别其实在于String是不可变的对象，字符串常量</li>
<li>StringBuffer和StringBuilder底层是char[]数组实现的，字符串变量</li>
<li>StringBuffer是线程安全的，而StringBuilder是线程不安全的</li>
<li>执行速度Stringbuilder-&gt;Stringbuffer-&gt;String</li>
</ul>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">String str  =  ”this is only a“ + &quot;simple&quot; + &quot;test&quot;;</span><br></pre></td></tr></table></figure>
<p>其实在内存中只创建了一个对象，等价于<br><figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">String str = “this is only a simple test”;</span><br></pre></td></tr></table></figure></p>
<h4 id="属性"><a href="#属性" class="headerlink" title="属性"></a>属性</h4><ul>
<li>private final char[] value</li>
<li>private int hash<br>缓存字符串的 hash Code，其默认值为 0</li>
<li>private static final long serialVersionUID = -6849794470754667710L<br>因为<code>String</code>实现了<code>Serializable</code>接口，所以支持序列化和反序列化。Java 的序列化机制是通过在运行时判断类的<code>serialVersionUID</code>来验证版本一致性的。在进行反序列化时，JVM 会把传来的字节流中的<code>serialVersionUID</code>与本地相应实体（类）的<code>serialVersionUID</code>进行比较，如果相同就认为是一致的，可以进行反序列化，否则就会出现序列化版本不一致的异常(<code>InvalidCastException</code>)</li>
</ul>
<h4 id="构造方法"><a href="#构造方法" class="headerlink" title="构造方法"></a>构造方法</h4><ul>
<li><p>使用字符串类型的对象来初始化</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="keyword">public</span> <span class="title">String</span><span class="params">(String original)</span></span>&#123;</span><br><span class="line">  <span class="keyword">this</span>.value = original.value;</span><br><span class="line">  <span class="keyword">this</span>.hash = original.hash;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>
<p>  直接将源<code>String</code>中的<code>value</code>和<code>hash</code>两个属性直接赋值给目标<code>String</code>。</p>
</li>
<li><p>空的构造方法</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="keyword">public</span> <span class="title">String</span><span class="params">()</span></span>&#123;</span><br><span class="line">  <span class="keyword">this</span>.value = <span class="string">""</span>.value;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>
<p>  这个构造方法会创建空的字符序列，这个构造方法会创造不必要的字符串对象，不建议采取。</p>
</li>
<li><p>使用字符数组来构造</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="keyword">public</span> <span class="title">String</span><span class="params">(<span class="keyword">char</span> value[], <span class="keyword">int</span> offset, <span class="keyword">int</span> count)</span></span>&#123;</span><br><span class="line">    <span class="keyword">if</span>(offset&lt;<span class="number">0</span>)&#123;</span><br><span class="line">        <span class="keyword">throw</span> <span class="keyword">new</span> StringIndexOutOfBoundsException(offset);</span><br><span class="line">    &#125;</span><br><span class="line">    <span class="keyword">if</span>(count&lt;=<span class="number">0</span>)&#123;</span><br><span class="line">        <span class="keyword">if</span>(count&lt;<span class="number">0</span>)&#123;</span><br><span class="line">            <span class="keyword">throw</span> <span class="keyword">new</span> <span class="function">String <span class="title">IndexOutOfBoundsException</span><span class="params">(count)</span></span>;</span><br><span class="line">        &#125;</span><br><span class="line">        <span class="keyword">if</span>(offset &lt;= value.length)&#123;</span><br><span class="line">            <span class="keyword">this</span>.value = <span class="string">""</span>.value;</span><br><span class="line">            <span class="keyword">return</span>;</span><br><span class="line">        &#125;</span><br><span class="line">  &#125;</span><br><span class="line">    <span class="keyword">if</span>(offset &gt; value.length - count)&#123;</span><br><span class="line">        <span class="keyword">throw</span> <span class="keyword">new</span> StringIndexOutOfBoundsException(offset+count);</span><br><span class="line">    &#125;</span><br><span class="line">    <span class="keyword">this</span>.value=Arrays.copyOfRange(value,offset,offset+count);</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>
<p>  对使用字符数组创建<code>String</code>的时候，会用到<code>Arrays.copyOf</code>方法或<code>Arrays.copyOfRange</code>方法。这两个方法是将原有的字符数组中的内容逐一的复制到<code>String</code>中的字符数组中。会创建一个新的字符串对象，随后修改的字符数组不影响新创建的字符串。</p>
</li>
<li><p>使用字节数组来构建<br>如果没有指明解码使用的字符集的话，那么<code>StringCoding</code>的<code>decode</code>方法首先调用系统的默认编码格式，如果没有指定编码格式则默认使用<code>ISO-8859-1</code>编码格式进行编码操作。</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="keyword">public</span> <span class="title">String</span><span class="params">(<span class="keyword">byte</span> bytes[])</span></span>&#123;</span><br><span class="line">    <span class="keyword">this</span>(bytes, <span class="number">0</span>, bytes.length);</span><br><span class="line">&#125;</span><br><span class="line"><span class="function"><span class="keyword">public</span> <span class="title">String</span><span class="params">(<span class="keyword">byte</span> bytes[], <span class="keyword">int</span> offset, <span class="keyword">int</span> length)</span></span>&#123;</span><br><span class="line">    checkBounds(bytes, offset, length);</span><br><span class="line">    <span class="keyword">this</span>.value = StringCoding.decode(bytes, offset, length);</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>
<p>  如果在使用<code>byte[]</code>构造<code>String</code>的时候，使用的是带有<code>charsetName</code>或者<code>charset</code>参数的构造方法，<code>String</code>内部就会使用<code>StringCoding.decode</code>方法进行解码，使用的解码的字符集就是我们指定的字符集。</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br></pre></td><td class="code"><pre><span class="line">String(<span class="keyword">byte</span> bytes[])</span><br><span class="line">String(<span class="keyword">byte</span> bytes[], <span class="keyword">int</span> offset, <span class="keyword">int</span> length)</span><br><span class="line">String(<span class="keyword">byte</span> bytes[], Charset charset)</span><br><span class="line">String(<span class="keyword">byte</span> bytes[], String charsetName)</span><br><span class="line">String(<span class="keyword">byte</span> bytes[], <span class="keyword">int</span> offset, <span class="keyword">int</span> length, Charset charset)</span><br><span class="line">String(<span class="keyword">byte</span> bytes[], <span class="keyword">int</span> offset, <span class="keyword">int</span> length, String charsetName)</span><br></pre></td></tr></table></figure>
<p>  解码方法:</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">static</span> <span class="keyword">char</span>[] decode(<span class="keyword">byte</span>[] ba, <span class="keyword">int</span> off, <span class="keyword">int</span> len)&#123;</span><br><span class="line">    String csn = Charset.defaultCharset().name();</span><br><span class="line">    <span class="keyword">try</span>&#123;</span><br><span class="line">        <span class="keyword">return</span> decode(csn, ba, off, len);</span><br><span class="line">    &#125; <span class="keyword">catch</span>(UnsupportedEncodingException x)&#123;</span><br><span class="line">        warnUnsupportedCharset(csn);</span><br><span class="line">    &#125;</span><br><span class="line">    <span class="keyword">try</span>&#123;</span><br><span class="line">        <span class="keyword">return</span> decode(<span class="string">"ISO-8859-1"</span>, ba, off, len);  </span><br><span class="line">    &#125; <span class="keyword">catch</span>(UnsupportedEncodingException x)&#123;</span><br><span class="line">        MessageUtils.err(<span class="string">"ISO-8859-1 char set not available: "</span> + x.toString());</span><br><span class="line">        System.exit(<span class="number">1</span>);</span><br><span class="line">        <span class="keyword">return</span> <span class="keyword">null</span>;</span><br><span class="line">    &#125;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>
</li>
<li><p>使用 StringBuffer 和 StringBuider 构造</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="keyword">public</span> <span class="title">String</span><span class="params">(StringBuffer buffer)</span> </span>&#123;</span><br><span class="line">    <span class="keyword">synchronized</span>(buffer) &#123;</span><br><span class="line">        <span class="keyword">this</span>.value = Arrays.copyOf(buffer.getValue(), buffer.length());</span><br><span class="line">    &#125; </span><br><span class="line">&#125; </span><br><span class="line"><span class="function"><span class="keyword">public</span> <span class="title">String</span><span class="params">(StringBuilder builder)</span> </span>&#123;</span><br><span class="line">    <span class="keyword">this</span>.value = Arrays.copyOf(builder.getValue(), builder.length());</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>
<p>  这两个构造方法是很少用到的，因为当我们有了<code>StringBuffer</code>或者<code>StringBuilfer</code>对象之后可以直接使用他们的<code>toString()</code> 方法来得到<code>String</code>。</p>
<p>  <strong>关于效率问题，Java 的官方文档有提到说使用S<code>tringBuilder</code>的<code>toString()</code>方法会更快一些，原因是<code>StringBuffer</code>的<code>toString</code>方法是<code>synchronized</code>的，在牺牲了效率的情况下保证了线程安全。</strong></p>
<p>  StringBuilder 的 toString() 方法：</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="keyword">public</span> String <span class="title">toString</span><span class="params">()</span></span>&#123;</span><br><span class="line">  <span class="keyword">return</span> <span class="keyword">new</span> String(value,<span class="number">0</span>,count);</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>
<p>  StringBuffer 的 toString() 方法：</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="keyword">public</span> <span class="keyword">synchronized</span> String <span class="title">toString</span><span class="params">()</span></span>&#123;</span><br><span class="line">    <span class="keyword">if</span> (toStringCache == <span class="keyword">null</span>)&#123;</span><br><span class="line">        toStringCache = Arrays.copyOfRange(value, <span class="number">0</span>, count);</span><br><span class="line">    &#125;</span><br><span class="line">    <span class="keyword">return</span> <span class="keyword">new</span> String(toStringCache, <span class="keyword">true</span>);</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>
</li>
</ul>
<h3 id="常用方法"><a href="#常用方法" class="headerlink" title="常用方法"></a>常用方法</h3><h4 id="截取字符串"><a href="#截取字符串" class="headerlink" title="截取字符串"></a>截取字符串</h4><figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="keyword">public</span> String <span class="title">substring</span><span class="params">(<span class="keyword">int</span> beginIndex, <span class="keyword">int</span> endIndex)</span></span>&#123;</span><br><span class="line">    <span class="keyword">if</span>(beginIndex &lt; <span class="number">0</span>)&#123;</span><br><span class="line">        <span class="keyword">throw</span> <span class="keyword">new</span> StringIndexOutOfBoundsException(beginIndex);</span><br><span class="line">    &#125;</span><br><span class="line">    <span class="keyword">if</span>(endIndex &gt; value.length)&#123;</span><br><span class="line">          <span class="keyword">throw</span> <span class="keyword">new</span> StringIndexOutOfBoundsException(endIndex);</span><br><span class="line">    &#125;</span><br><span class="line">    <span class="keyword">int</span> subLen = endIndex-beginIndex;</span><br><span class="line">    <span class="keyword">if</span>(subLen &lt; <span class="number">0</span>)&#123;</span><br><span class="line">        <span class="keyword">throw</span> <span class="keyword">new</span> StringIndexOutOfBoundsException(subLen);</span><br><span class="line">    &#125;</span><br><span class="line">    <span class="keyword">return</span> ((beginIndex == <span class="number">0</span>) &amp;&amp; (endIndex == value.length)) ? <span class="keyword">this</span>  : newString(value, beginIndex, subLen);</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>
<p>创建一个新的<code>String</code>并返回，这个方法会将原来的<code>char[]</code>中的值逐一复制到新的<code>String</code>中，两个数组并不是共享的，虽然这样做损失一些性能，但是有效地避免了内存泄露。</p>
<h4 id="返回字符串是否为空"><a href="#返回字符串是否为空" class="headerlink" title="返回字符串是否为空"></a>返回字符串是否为空</h4><figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="keyword">public</span> <span class="keyword">boolean</span> <span class="title">isEmpty</span><span class="params">()</span></span>&#123;</span><br><span class="line">    <span class="keyword">return</span> value.length == <span class="number">0</span>;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>
<h4 id="返回字符串中第（index-1）个字符（数组索引）"><a href="#返回字符串中第（index-1）个字符（数组索引）" class="headerlink" title="返回字符串中第（index+1）个字符（数组索引）"></a>返回字符串中第（index+1）个字符（数组索引）</h4><figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="keyword">public</span> <span class="keyword">char</span> <span class="title">charAt</span><span class="params">(<span class="keyword">int</span> index)</span></span>&#123;</span><br><span class="line">    <span class="keyword">if</span>((index &lt; <span class="number">0</span>) || (index &gt;= value.length))&#123;</span><br><span class="line">        <span class="keyword">throw</span> <span class="keyword">new</span> StringIndexOutOfBoundsException(index);</span><br><span class="line">    &#125;</span><br><span class="line">    <span class="keyword">return</span> value[index];</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>
<h4 id="将一个字符串转换成字节数组"><a href="#将一个字符串转换成字节数组" class="headerlink" title="将一个字符串转换成字节数组"></a>将一个字符串转换成字节数组</h4><figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">public</span> <span class="keyword">byte</span>[] getBytes()&#123;</span><br><span class="line">    <span class="keyword">return</span> StringCoding.encode(value, <span class="number">0</span>, value.length);</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>
<p>这个方法在不同的平台上运行得到结果是不一样的。由于没有指定编码方式，所以在该方法对字符串进行编码的时候就会使用系统的默认编码方式。<br><strong>在中文操作系统中可能会使用 GBK 或者 GB2312 进行编码，在英文操作系统中有可能使用 iso-8859-1 进行编码。</strong><br>这样写出来的代码就和机器环境有很强的关联性了，为了避免不必要的麻烦，要指定编码方式：<br><figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">public</span> <span class="keyword">byte</span>[] getBytes(String charsetName) <span class="keyword">throws</span> UnsupportedEncodingException&#123;</span><br><span class="line">    <span class="keyword">if</span> (charsetName == <span class="keyword">null</span>) <span class="keyword">throw</span> <span class="keyword">new</span> NullPointerException();</span><br><span class="line">    <span class="keyword">return</span> StringCoding.encode(charsetName, value, <span class="number">0</span>, value.length);</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure></p>
<h4 id="比较方法"><a href="#比较方法" class="headerlink" title="比较方法"></a>比较方法</h4><ul>
<li><p>boolean equals(Object anObject)</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="keyword">public</span> <span class="keyword">boolean</span> <span class="title">equals</span><span class="params">(Object anObject)</span> </span>&#123;</span><br><span class="line">    <span class="keyword">if</span> (<span class="keyword">this</span> == anObject) &#123;</span><br><span class="line">        <span class="keyword">return</span> <span class="keyword">true</span>;</span><br><span class="line">    &#125; </span><br><span class="line">    <span class="keyword">if</span> (anObject <span class="keyword">instanceof</span> String) &#123;</span><br><span class="line">        String anotherString = (String) anObject;</span><br><span class="line">        <span class="keyword">int</span> n = value.length;</span><br><span class="line">        <span class="keyword">if</span> (n == anotherString.value.length) &#123;</span><br><span class="line">            <span class="keyword">char</span> v1[] = value;</span><br><span class="line">            <span class="keyword">char</span> v2[] = anotherString.value;</span><br><span class="line">            <span class="keyword">int</span> i = <span class="number">0</span>;</span><br><span class="line">            <span class="keyword">while</span> (n-- != <span class="number">0</span>) &#123;</span><br><span class="line">                <span class="keyword">if</span> (v1[i] != v2[i])</span><br><span class="line">                    <span class="keyword">return</span> <span class="keyword">false</span>;</span><br><span class="line">                i++;</span><br><span class="line">            &#125;</span><br><span class="line">            <span class="keyword">return</span> <span class="keyword">true</span>;</span><br><span class="line">        &#125;</span><br><span class="line">    &#125; </span><br><span class="line">    <span class="keyword">return</span> <span class="keyword">false</span>;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>
<p>  该方法首先判断this == anObject ？，也就是说判断要比较的对象和当前对象是不是同一个对象，如果是直接返回 true，如不是再继续比较，然后在判断anObject 是不是 String类型的，如果不是，直接返回 false，如果是再继续比较，到了能终于比较字符数组的时候，先比较两个数组的长度，不一样直接返回 false，一样再逐一比较值。 虽然代码写的内容比较多，但是可以很大程度上提高比较的效率。</p>
</li>
<li><p>boolean equalsIgnoreCase(String anotherString)<br>忽略大小写进行比较</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="keyword">public</span> <span class="keyword">boolean</span> <span class="title">equalsIgnoreCase</span><span class="params">(String anotherString)</span> </span>&#123;</span><br><span class="line">    <span class="keyword">return</span> (<span class="keyword">this</span> == anotherString) ? <span class="keyword">true</span> : (anotherString != <span class="keyword">null</span>) </span><br><span class="line">      &amp;&amp; (anotherString.value.length == value.length) </span><br><span class="line">      &amp;&amp; regionMatches(<span class="keyword">true</span>, <span class="number">0</span>, anotherString, <span class="number">0</span>, value.length);</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>
</li>
</ul>
<h4 id="replaceFirst、replaceAll、replace区别"><a href="#replaceFirst、replaceAll、replace区别" class="headerlink" title="replaceFirst、replaceAll、replace区别"></a>replaceFirst、replaceAll、replace区别</h4><figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line"><span class="function">String <span class="title">replaceFirst</span><span class="params">(String regex, String replacement)</span></span></span><br><span class="line"><span class="function">String <span class="title">replaceAll</span><span class="params">(String regex, String replacement)</span></span></span><br><span class="line"><span class="function">String <span class="title">replace</span><span class="params">(Char Sequencetarget, Char Sequencereplacement)</span></span></span><br></pre></td></tr></table></figure>
<ul>
<li>replace 的参数是 char 和 CharSequence，即可以支持字符的替换, 也支持字符串的替换</li>
<li>replaceAll 和 replaceFirst 的参数是 regex，即基于规则表达式的替换<br>比如可以通过 replaceAll (“\d”, “*”) 把一个字符串所有的数字字符都换成星号;</li>
<li>如果replaceAll() 和r eplaceFirst() 所用的参数据不是基于规则表达式的，则与replace()替换字符串的效果是一样的</li>
</ul>
<h4 id="String对“-”的重载"><a href="#String对“-”的重载" class="headerlink" title="String对“+”的重载"></a>String对“+”的重载</h4><figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line">String string=<span class="string">"hollis"</span>;</span><br><span class="line">String string2 = string + <span class="string">"chuang"</span>;</span><br></pre></td></tr></table></figure>
<p>这段代码的实际执行情况：</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line">String string = <span class="string">"hollis"</span>;</span><br><span class="line">String string2 = (<span class="keyword">new</span> StringBuilder(String.valueOf(string))).append(<span class="string">"chuang"</span>).toString();</span><br></pre></td></tr></table></figure>
<p>其实<code>String</code>对“+”的支持其实就是使用了<code>StringBuilder</code>以及他的<code>append</code>、<code>toString</code>两个方法。</p>
<h4 id="String-valueOf和Integer-toString的区别"><a href="#String-valueOf和Integer-toString的区别" class="headerlink" title="String.valueOf和Integer.toString的区别"></a>String.valueOf和Integer.toString的区别</h4><figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">int</span> i = <span class="number">5</span>;</span><br><span class="line">String i1 = <span class="string">""</span> + i;</span><br><span class="line">String i2 = String.valueOf(i);</span><br><span class="line">String i3 = Integer.toString(i);</span><br></pre></td></tr></table></figure>
<p>第二行代码其实是<code>String i1 = (new StringBuilder()).append(i).toString();</code><br>第三行和第四行没有任何区别，因为<code>String.valueOf(i)</code>也是调用<code>Integer.toString(i)</code>来实现的。</p>
<h3 id="switch对字符串支持的实现"><a href="#switch对字符串支持的实现" class="headerlink" title="switch对字符串支持的实现"></a>switch对字符串支持的实现</h3><p>先看代码：</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br></pre></td><td class="code"><pre><span class="line">  <span class="function"><span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title">main</span><span class="params">(String[] args)</span> </span>&#123;</span><br><span class="line">    String str = <span class="string">"world"</span>;</span><br><span class="line">    <span class="keyword">switch</span> (str) &#123;</span><br><span class="line">    <span class="keyword">case</span> <span class="string">"hello"</span>: </span><br><span class="line">        System.out.println(<span class="string">"hello"</span>);</span><br><span class="line">        <span class="keyword">break</span>;</span><br><span class="line">    <span class="keyword">case</span> <span class="string">"world"</span>:</span><br><span class="line">        System.out.println(<span class="string">"world"</span>);</span><br><span class="line">        <span class="keyword">break</span>;</span><br><span class="line">    <span class="keyword">default</span>: <span class="keyword">break</span>;</span><br><span class="line">    &#125;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>
<p>反编译之后：</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title">main</span><span class="params">(String args[])</span> </span>&#123;</span><br><span class="line">    String str = <span class="string">"world"</span>;</span><br><span class="line">    String s;</span><br><span class="line">    <span class="keyword">switch</span>((s = str).hashCode()) &#123;</span><br><span class="line">    <span class="keyword">case</span> <span class="number">99162322</span>:</span><br><span class="line">        <span class="keyword">if</span>(s.equals(<span class="string">"hello"</span>))</span><br><span class="line">            System.out.println(<span class="string">"hello"</span>);</span><br><span class="line">        <span class="keyword">break</span>;</span><br><span class="line">    <span class="keyword">case</span> <span class="number">113318802</span>:</span><br><span class="line">        <span class="keyword">if</span>(s.equals(<span class="string">"world"</span>))</span><br><span class="line">            System.out.println(<span class="string">"world"</span>);</span><br><span class="line">        <span class="keyword">break</span>;</span><br><span class="line">    <span class="keyword">default</span>: </span><br><span class="line">        <span class="keyword">break</span>;</span><br><span class="line">    &#125;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>
<p><strong>原来字符串的<code>switch</code>是通过<code>equals()</code>和<code>hashCode()</code>方法来实现的。</strong><br><code>switch中</code>只能使用整型，比如<code>byte</code>，<code>short</code>，<code>char</code>(ackii码是整型)以及<code>int</code>。而<code>hashCode()</code>方法返回的正好是<code>int</code>而不是<code>long</code>。<br>进行<code>switch</code>的实际是哈希值，然后通过使用<code>equals()</code>方法比较进行安全检查，这个检查是必要的，因为哈希可能会发生碰撞。因此性能是不如使用枚举进行<code>switch</code>或者使用纯整数常量。<br>其实<code>switch</code>只支持一种数据类型，那就是整型，其他数据类型都是转换成整型之后在使用<code>switch</code>的。</p>
<h3 id="字符串的不可变性"><a href="#字符串的不可变性" class="headerlink" title="字符串的不可变性"></a>字符串的不可变性</h3><p>字符串池是方法区中的一部分特殊存储。当一个字符串被被创建的时候，首先会去这个字符串池中查找，如果找到，直接返回对该字符串的引用。<br>字符串的为什么要被设计为不可变性呢？</p>
<ul>
<li>缓存Hashcode<br>Java中经常会用到字符串的哈希码（<code>hashcode</code>）。例如，在<code>HashMap</code>中，字符串的不可变能保证其<code>hashcode</code>永远保持一致，这样就可以避免一些不必要的麻烦。这也就意味着每次在使用一个字符串的hashcode的时候不用重新计算一次，这样更加高效。</li>
<li>安全性<br><code>String</code>被广泛的使用在其他Java类中充当参数。比如网络连接、打开文件等操作。如果字符串可变，那么类似操作可能导致安全问题。</li>
<li>不可变对象天生就是线程安全的<br>因为不可变对象不能被改变，所以他们可以自由地在多个线程之间共享。不需要任何同步处理。</li>
</ul>
<h3 id="总结"><a href="#总结" class="headerlink" title="总结"></a>总结</h3><ul>
<li>一旦<code>string</code>对象在内存(堆)中被创建出来，就无法被修改<br>特别要注意的是，<code>String</code>类的所有方法都没有改变字符串本身的值，都是返回了一个新的对象。</li>
<li>如果你需要一个可修改的字符串，应该使用<code>StringBuffer</code>或者<code>StringBuilder</code><br>否则会有大量时间浪费在垃圾回收上，因为每次试图修改都有新的string对象被创建出来。</li>
<li>如果你只需要创建一个字符串，你可以使用双引号的方式，如果你需要在堆中创建一个新的对象，你可以选择构造函数的方式</li>
</ul>

      
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              <div class="post-toc-content"><ol class="nav"><li class="nav-item nav-level-3"><a class="nav-link" href="#String类的定义"><span class="nav-number">1.</span> <span class="nav-text">String类的定义</span></a><ol class="nav-child"><li class="nav-item nav-level-4"><a class="nav-link" href="#属性"><span class="nav-number">1.1.</span> <span class="nav-text">属性</span></a></li><li class="nav-item nav-level-4"><a class="nav-link" href="#构造方法"><span class="nav-number">1.2.</span> <span class="nav-text">构造方法</span></a></li></ol></li><li class="nav-item nav-level-3"><a class="nav-link" href="#常用方法"><span class="nav-number">2.</span> <span class="nav-text">常用方法</span></a><ol class="nav-child"><li class="nav-item nav-level-4"><a class="nav-link" href="#截取字符串"><span class="nav-number">2.1.</span> <span class="nav-text">截取字符串</span></a></li><li class="nav-item nav-level-4"><a class="nav-link" href="#返回字符串是否为空"><span class="nav-number">2.2.</span> <span class="nav-text">返回字符串是否为空</span></a></li><li class="nav-item nav-level-4"><a class="nav-link" href="#返回字符串中第（index-1）个字符（数组索引）"><span class="nav-number">2.3.</span> <span class="nav-text">返回字符串中第（index+1）个字符（数组索引）</span></a></li><li class="nav-item nav-level-4"><a class="nav-link" href="#将一个字符串转换成字节数组"><span class="nav-number">2.4.</span> <span class="nav-text">将一个字符串转换成字节数组</span></a></li><li class="nav-item nav-level-4"><a class="nav-link" href="#比较方法"><span class="nav-number">2.5.</span> <span class="nav-text">比较方法</span></a></li><li class="nav-item nav-level-4"><a class="nav-link" href="#replaceFirst、replaceAll、replace区别"><span class="nav-number">2.6.</span> <span class="nav-text">replaceFirst、replaceAll、replace区别</span></a></li><li class="nav-item nav-level-4"><a class="nav-link" href="#String对“-”的重载"><span class="nav-number">2.7.</span> <span class="nav-text">String对“+”的重载</span></a></li><li class="nav-item nav-level-4"><a class="nav-link" href="#String-valueOf和Integer-toString的区别"><span class="nav-number">2.8.</span> <span class="nav-text">String.valueOf和Integer.toString的区别</span></a></li></ol></li><li class="nav-item nav-level-3"><a class="nav-link" href="#switch对字符串支持的实现"><span class="nav-number">3.</span> <span class="nav-text">switch对字符串支持的实现</span></a></li><li class="nav-item nav-level-3"><a class="nav-link" href="#字符串的不可变性"><span class="nav-number">4.</span> <span class="nav-text">字符串的不可变性</span></a></li><li class="nav-item nav-level-3"><a class="nav-link" href="#总结"><span class="nav-number">5.</span> <span class="nav-text">总结</span></a></li></ol></div>
            

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